Not just in BDSM, but that, and sex in general. We all have hard limits, those that we just WILL NOT do.
I'm one of the most open minded, and experimental people I know. Pervert, and proud. There are things that make me cringe.
Without getting into children, animals, or dead people (for obvious reasons) What are your hard limits?
Me- An activity likely to make me bleed. Marks/bruises anywhere but my buttocks, Breath deprivation/ strangling. (put your hands around my neck, and you're looking for an assault charge, AFTER I kick you in the nuts), rape fantasies. Scat (a little poop during anal doesn't scare me, but poop as any form of sex...no)